Aircraft that have arrays of photovoltaic solar cells (referred to as solar panels) on the aircraft's wings for converting solar energy into electrical power are known. The electrical power may be used to operate on-board aircraft systems and/or to propel the aircraft, typically using one or more electric motors and propellers. The solar panels are typically mounted on upper surfaces of the wings because this location provides favorable exposure to sunlight and provides a relatively large area that is sufficient for mounting the number and size of the solar panels that are needed for the application.
Designing solar powered aircraft, particularly those flying at high altitudes, has several challenges. Because the solar energy collected by the panels is relatively low in density, the amount of solar generated power available for propulsion of the aircraft is relatively small, which in turn dictates that the aircraft have a relatively large wingspan and be very lightweight. However, long, lightweight wings may be highly flexible, presenting a number of problems due to the loads imposed on the wings during flight. For example, typical lightweight solar panels have a foundation of copper, rendering them relatively stiff with a relatively high coefficient of thermal expansion (CTE). Flexing of the wings during flight may cause the solar panels to experience high strains that can adversely affect the integrity of the solar panels. These strains may also result in buckling or wrinkling of the solar panels which may trip the airflow boundary layer over the wings from laminar to turbulent, or cause air flow separation, which may significantly reduce the aerodynamic performance of the aircraft.
The integrity of the solar panels and/or the aerodynamic performance of the wings may also be adversely affected by large temperature swings experienced by the aircraft during flight. These temperature swings may result in excessive stresses and strains between the solar panel and the wing structure, due to differences in thermal expansion.
The above-cited U.S. application Ser. No. 13/276,750 discloses a flexible, lightweight, solar panel equipped wing that substantially isolates the solar panels from strains experienced during flight due to wing flexing and/or differences in thermal expansion between the components of the wing. There remains a need, however, for a way to provide lightweight stiffening to the skin of solar powered aircraft that use photovoltaic solar cells mounted on the surface of the wings to operate aircraft systems.